Ldpe recycle system defouling method

ABSTRACT

A series of recycle coolers in a high-pressure polyethylene system are defouled by circulating ethylene feed using a compressor through the polyethylene system without addition of initiator and through the series of recycle coolers and maintaining during the sequential heating of each of said series of recycle coolers a temperature at the inlet of the compressor of the circulating feed less than the design temperature of the compressor which may be between about 150* to 225* F.

United States Patent [72] Inventors Charles D. Beals;

George I. Fitzpatrick; Kim L. O'Hara, all of Baton Rouge, La. [21] Appl.No. 41,242 [22] Filed May 25, 1970 [45] Patented Dec. 14, 1971 [73]Assignee Esso Research and Engineering Company [54] LDPE RECYCLE SYSTEMDEFOULING METHOD 5 Claims, 1 Drawing Fig.

[52] US. Cl 260/943 P, 260/949 R [5 l 1 Int. Cl C08! 3/04 [50] Field ofSearch 260/949 P, 94.9 R

[56] References Cited UNITED STATES PATENTS 2,396,791 3/ l 946 Kraseeta] MW 269/949 High Tempg Cooler Reactor Effluent Hi h PenguinSoparulor v Cooling Media Polymer lo Finishing 3,306,889 2/1967Schappert 260/949 Primary Examiner-Joseph L. Schofer AssistantExaminer-A. Holler Attorneys-Thomas B. McCulloch, Melvin F. Fincke,Timothy L. Burgess, John S. Schneider, Sylvester W. Brock, Jr. and KurtS. Myers ABSTRACT: A series of recycle coolers in a high-pressurepolyethylene system are defouled by circulating ethylene feed using acompressor through the polyethylene system without addition of initiatorand through the series of recycle coolers and maintaining during thesequential heating of each of said series of recycle coolers atemperature at the inlet of the compressor of the circulating'feed lessthan the design temperature of the compressor which may be between about150 to Chilled Ethylene o Compressor n 3;. Knock-out 13 Knock-out Temp.Cooler E Cooler 7 Cooling Medic LDPE RECYCLE SYSTEM DEFOULING METHODBACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention is directed to a method for defouling a series of recyclecoolers in a high-pressure polyethylene system.

2. Prior Art Heretofore, the defouling method of recycle coolers in ahigh-pressure polyethylene system has occurred as the ethylene is beingpolymerized. Such in-line defouling method is ineffectual in sustainingimproved recycle cooler performance in that the lowest temperaturerecycle cooler can not be effectively defouled since it is noteconomically feasible to design a polyethylene secondary compressor tooperate at design discharge pressures and capacities and simultaneouslywith high suction temperatures (150-225 F.). Other defouling proceduresinvolve mechanically cleaning tubes of the recycle coolers while thehigh-pressure polyethylene system is shut down.

It has now been discovered that an improved defouling method of therecycle coolers may be used utilizing the circulating ethylene feed withno polymerization occuring. The method may be carried out advantageouslyjust prior to shutting down or starting up the polyethylene system.

SUMMARY OF THE INVENTION The present invention may be briefly describedas a method for defouling a series of recycle coolers in a high-pressurepolyethylene system. Ethylene feed is circulated through the reactor inthe polyethylene system without addition of initiator and, accordingly,without polymerization occurring and through the series of recyclecoolers. The first in the series of recycle coolers is heatedsufficiently to remove polymer from that recycle cooler. The temperaturerequired is higher than is required for the following recycle coolers.The polymer is collected in a knockout pot following each recyclecooler. Each of the remaining recycle coolers is sequentially heated toremove the polymer as the defouled recycle coolers are placed back inservice for the necessary cooling of the recycled ethylene feed. Theethylene feed stream is maintained during the defouling of the last ofthe series of recycle coolers was to maintain a temperature at the inletof the compressor between about 150 to 250 F. Preferably, each of therecycle coolers is heated by steam to obtain the desired heating in thecooler in the shortest period of time.

DESCRIPTION OF THE DRAWING The present invention will be furtherillustrated by reference to the drawing in which:

The single FIGURE illustrates a series of recycle coolers in ahigh-pressure polyethylene system.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing,the reactor effluent is introduced by line into a high-pressureseparator 11. The gases are removed by line 12 for recycling while thepolyethylene is removed by line 8 through let down valve 9 to finishing.In the recycle line 12 are a series of recycle coolers and knockout potsfor cooling and dewaxing the recycle stream before introducing it into acompressor (not shown). The cooling is done in stages through a seriesof recycle coolers illustrated by coolers 13, 14, and 15. A series ofrecycle coolers is used since fouling is most severe at largetemperature drops.

The recycle cooler 13 is provided with cooling media by line 16 and anoutlet line 17 wherein hot water, for example, may be circulated. In theintermediate temperature recycle cooler 14, cooling media is introducedby line 18 and is removed by line 19, and cooling water, for example,may be used as the cooling media. In the low-temperature recycle cooler15, cooling media is circulated through line 20 and removed by line 21and chilled water, for example, may be used. Following each of therecycle coolers 13, 14, and 15 is a knockout pot for collecting polymer.Knockout pot 22 follows recycle cooler 13, knockout pot 23 followsrecycle cooler 14 and knockout pot 24 follows recycle cooler 15. Each ofthe recycle coolers 13, 14, and 15 may be standard elevated pressuretube bundle heat exchangers with the recycle gases passing through thetubes and the cooling media circulating through the shell side of theheat exchanger. It is preferred, according to the present invention, toprovide lines 25, 26, and 27 to each of the recycle coolers 13, 14, and15, respectively, so that steam may be circulated or introduced to theshell side of the heat exchanger in place of the cooling media.

According to the present invention, the ethylene feed is circulated bythe compressor (not shown) through the reactor in the polyethylenesystem and into the reactor efiluent stream 10 and into thehigh-pressure separator 11. The reactor, such as a tubular reactor or anautoclave reactor, is operated with maximum heating on all jacketsections to maintain the highpressure separator 1 1 gas temperature atabout 350 to 450 F. The reactor pressure is minimized by operating withthe highpressure letdown valve (not shown) open. Accordingly, thepressure in the polyethylene system is less than l0,000 p.s.i.,preferably.

The first of the recycle coolers 14 may be heated for defouling byblocking the circulation of cooling media by closing the valves in lines16 and 17. It is preferred, however, to heat the cooler by draining thecooler 13 of the heat exchange media and introducing steam by line 25.The ethylene feed is circulated for about 10 minutes as the temperaturein recycle cooler 13 is raised. The polymer which has collected incooler 13 is removed and collected in knockout pot 22. The temperatureof the circulating ethylene feed is reduced in recycle coolers l4 and 15sufficiently so that the temperature at the inlet to the compressor doesnot exceed about 250 F.

After the recycle cooler 13 has been defouled, it is placed back inservice by again circulating the cooling media through lines 16 and 17in sufiicient amount so as to aid in the cooling of the recycle feed.Suitable exit temperature from the defouled cooler 13 may be between 150to 300 F.

Recycle cooler 14 is then heated by either blocking in the cooling mediaby closing the valves in lines 18 and 19 or, preferably, the cooler 14is drained and steam introduced by line 26. The recycle ethylene feed iscontinued for about [0 minutes as the intermediate temperature recyclecooler 14 is heated. The polymer which is removed therefrom is collectedin knockout pot 23. Thereafter, the recycle cooler 14 is placed back inservice by again circulating cooling media through lines 18 and 19 insufficient amount so as to aid in the cooling of the recycle ethylenefeed. Suitable temperatures of the recycle ethylene feed, as it leavesdefouled recycle cooler 14, may be about 150 to 225 F.

Recycle cooler 15 is then heated by either blocking in the cooling mediaby closing the valves in lines 20 and 21 or, preferably, the cooler 15is drained and steam introduced through line 27. The temperature isincreased in recycle cooler 15 sufficiently to remove the polymertherefrom and is collected in knockout pot 24. The temperature of thecirculating ethylene feed is controlled such that the temperature at theinlet of the compressor is such that the ethylene feed will not exceedthe design conditions of the compressor as the feed is circulated therethrough. This may suitably be at inlet temperatures between 150 and 225F.

After all of the recycle coolers are defouled by the foregoing method,the temperature of the ethylene feed as it is recycled to the compressorin the polyethylene system and polymerization or normal operation isresumed may be within the range of 60 to 120 F. and, preferably, as lowas 60 to F. By having all the recycle coolers clean of polymer andoperating at their maximum efficiency, the inlet temperature to thecompressor may be reduced just after defouling 20 to 50 F. and, on theaverage, approximately 10 to 25 F. before again defouling and,therefore, increases the compressor capacity.

The nature and object of the present invention having been completelydescribed and illustrated and the best mode contemplated set forth, whatwe wish to claim as new and useful and secure by Letters Patents is:

l. A method for defouling a series of recycle coolers in a high-pressurepolyethylene system comprising a reactor in a series of steps whichcomprises:

circulating gaseous ethylene feed by using a compressor through thereactor in said polyethylene system without addition of initiator and inthe absence of polymerization and then through said series of recyclecoolers, heating by indirect heat exchange the first of said series ofrecycle coolers sufficiently to remove polymer from that heated recyclecooler as gaseous feed is circulated therethrough at a temperature ofabout 350 to 450 F.

collecting polymer from said circulating feed after passing through saidfirst heated recycle cooler,

sequentially heating by indirect heat exchange each of the remaining ofsaid series of recycle coolers as gaseous feed is circulatedtherethrough and collecting the polymer therefrom while resuming coolingin each previously heated recycle cooler, and

maintaining during the sequential heating of each of said series ofrecycle coolers a temperature at the inlet of said compressor of saidcirculating feed between about [50 to 225 F.

2. A method according to claim 1 wherein said polyethylene reactor is atubular reactor.

3. A method according to claim 1 wherein said polyethylene reactor is anautoclave reactor.

4. A method according to claim 1 wherein said increasing of temperatureis obtained by blocking-off coolant.

5. A method according to claim 4 wherein said increasing of temperatureis further obtained by draining the cooler and the addition of steam.

* l l l

2. A method according to claim 1 wherein said polyethylene reactor is atubular reactor.
 3. A method according to claim 1 wherein saidpolyethylene reactor is an autoclave reactor.
 4. A method according toclaim 1 wherein said increasing of temperature is obtained byblocking-off coolant.
 5. A method according to claim 4 wherein saidincreasing of temperature is further obtained by draining the cooler andthe addition of steam.